In communications systems, a transmitter sends data streams to a receiver in symbols, such as bits of data. As the receiver clock is typically not synchronized with the transmitter clock, the receiver needs to correctly recover the clock from the received signal itself. In addition, when data is transmitted over a communication channel, it is usually distorted in terms of phase and amplitude due to various types of noise, such as fading, oscillator drift, frequency and phase offset, and receiver thermal noise. At the receiver, the system is also subject to noise and timing jitter in a time domain. Therefore, the receiver needs a timing recovery process to obtain symbol synchronization, particularly to correct the clock delay and derive the optimal clock phase that is used to sample the received signal and achieve the best Signal-to-Noise Ratio (SNR).
Commonly, in an equalizer-based timing recovery loop of a receiver, an adaptive equalizer is employed to mitigate the effect of intersymbol interference (ISI) caused by the channel distortion. An adaptive equalizer enables the equalization process to be adapted to changes in channel characteristics over time. Typically, the adaptation is performed by dynamically adapting equalization parameters, such as the tap weights of an equalization filter.
Adaptive equalization itself can also result in correction for time delays of the input clock embedded in the received signal, which undesirably interferes with the clock recovery process by the overall timing recovery loop, e.g., to be performed by a phase detector, a loop filter and a VCO in the same timing recovery loop. Particularly, the interference may cause recovered data symbols to shift from their optimized locations. Thus, it is desirable that only the timing recovery loop corrects for delay of the input clock in the received signal.
Conventionally, the problematic interaction between an adaptive equalizer and the associated timing recovery loop is solved by freezing or slowing down the adaptation of the equalizer once acquisition is achieved. More specifically, all the tap weights of the equalization filter are fixed or adapted in small steps to prevent the interaction with the timing recovery loop regarding time delay correction. Unfortunately, freezing or slowing down the equalizer adaptation process inevitably impairs the equalizer's capability of tracking changes in channel characteristics over time.